1. Field of the Invention
The present invention relates to a pattern inspection apparatus and a pattern inspection method. For example, it relates to an inspection apparatus and method for inspecting a pattern using pulsed lights.
2. Description of Related Arts
In recent years, with an increase in high integration and large capacity of a large-scale integrated circuit (LSI), a circuit line width required for semiconductor elements is becoming narrower and narrower. These semiconductor elements are manufactured by exposing and transferring a pattern onto a wafer to form a circuit by means of a reduced projection exposure apparatus (a so-called stepper) while using a master or “original” pattern (also called a mask or a reticle, and hereinafter generically called a mask) on which a circuit pattern is written, “drawn” or “formed”. Therefore, in order to manufacture a mask for transfer printing a fine circuit pattern onto a wafer, an electron beam pattern writing apparatus capable of writing a fine circuit pattern needs to be employed. The pattern circuit may be directly written onto a wafer by the pattern writing apparatus. In addition to the writing apparatus using electron beams, a laser beam writing apparatus which uses laser beams to write a pattern is also under development.
Since a lot of manufacturing cost is needed for the production of LSI, an improvement in yield is a crucial issue. However, as typified by a DRAM (Dynamic Random Access Memory) of 1 giga-bit class, the order of a pattern constituting the LSI has been changing from submicron to nano-meter. Then, one of major factors that decrease the yield is a pattern defect of a mask used in exposing and transferring an ultrafine pattern onto a semiconductor wafer by a photolithography technique. In recent years, with miniaturization of an LSI pattern formed on a semiconductor wafer, dimensions to be detected as a pattern defect have become extremely small. Therefore, a pattern inspection apparatus for inspecting defects of a transfer mask used in manufacturing the LSI needs to be highly accurate.
On the other hand, with development of multimedia technologies, the size of a liquid crystal substrate of an LCD (Liquid Crystal Display) is becoming large, e.g., 500 mm×600 mm or more, and a pattern of a TFT (Thin Film Transistor) or the like formed on the liquid crystal substrate is becoming minute. Therefore, it is increasingly required to inspect an ultra-fine pattern defect in a large area. For this reason, development of a pattern inspection apparatus which, in a short time, efficiently inspects defects of a pattern of a large-area LCD and a photomask used in manufacturing the large-area LCD is urgently required.
As to a conventional pattern inspection apparatus, it is well-known that inspecting is performed by comparing an optical image captured by photographing a pattern formed on a target workpiece or “sample”, such as a lithography mask, at a predetermined magnification by use of a magnification optical system with design data or an optical image captured by photographing the same pattern in a different region on the target workpiece. For example, the following is known as pattern inspection methods: “die to die inspection” that compares optical image data obtained by capturing images of the same patterns at different positions on the same mask, and “die to data base inspection” that performs inputting writing data (design pattern data), which is generated by converting pattern CAD data into an appropriate format to be input by a writing apparatus when writing a pattern on a mask, into an inspection apparatus, generating design image data (reference image) based on the inputted writing data, and comparing the design image data with an optical image serving as measurement data obtained by capturing the image of the pattern. In the inspecting methods of the inspection apparatus, the target workpiece is placed on a stage to be scanned by a flux of light while the stage is moving to perform inspection. The target workpiece is irradiated with a flux of light from a light source and an irradiation optical system. Light transmitted through the target workpiece or reflected by the target workpiece is focused on a sensor through the optical system. The image captured by the sensor is transmitted to a comparison circuit as measurement data. In the comparison circuit, after position alignment of the images, the measurement data and the reference data are compared based on an appropriate algorithm. When the measurement data is different from the reference data, it is judged that there is a pattern defect (refer to, e.g., Japanese Unexamined Patent Publication No. 2007-102153 (JP-A-2007-102153)).
Conventionally, continuous light is used as irradiation light. In order to detect finer defects, it is necessary to use a light of short wavelength in the pattern inspection apparatus. As the light of short wavelength, a KrF excimer laser with a wavelength of 248 nm or an ArF excimer laser with a wavelength of 193 nm can be exemplified. However, the excimer laser is a pulsed laser. Moreover, in recent years, although new type of laser apparatuses that consist of only solid state lasers and are capable of emitting a laser of 193 nm have been produced, they use pulsed lasers. That is, lights of short wavelengths are pulsed laser lights in many cases as stated above. Such pulsed lasers oscillate at a frequency of 1 kHz to several MHz. Moreover, emission of only about several (n) seconds per pulse can be obtained. Furthermore, a light intensity difference of about 30% is generated per pulse light, thereby a large error occurs in the measured light quantity.
Conventionally, as light for illuminating a target workpiece, continuous light has been used. That is, the target workpiece is always illuminated to be in a bright state. Then, in this state, an optical image is captured while the stage is continuously moving at a fixed speed, using a line sensor of one-dimensional array which receives an image in one dimensional direction at a time, as a sensor for measuring a quantity of light of a pixel, for example. A region inspected during once continuous movement of the stage is hereinafter called a frame. Alternatively, instead of the one-dimensional line sensor, there is a case of using a sensor (TDI sensor) in which one thousand light receiving elements are arrayed perpendicularly to the stage movement and about five light receiving elements are arrayed in the movement direction. Each light receiving element of the TDI sensor measures light quantity during a predetermined time period and sends the measurement result to an adjoining light receiving element in the movement direction of the stage. The adjoining light receiving element adds a light quantity measured by itself during a predetermined time to the received measurement result, and sends the addition result to a further adjoining light receiving element. After repeating this, the total of the measured light quantity is output from the light receiving elements in the last row. By virtue of this, information on one pixel on the target workpiece is measured as the sum of the light intensity measured by the five light receiving sensors. In these conventional methods, it takes about two hours, for example, to inspect one target workpiece. In any case, it is the premise that continuous light is used and the measurement region of the target workpiece is always illuminated to be in a bright state. Thereby, by utilizing this, the quantity of light from each region is measured to inspect defects.
However, if these methods are intact, they cannot be used for a pulsed laser. FIG. 18 shows an example of a photoperiod and a light quantity of a pulsed laser light source. In FIG. 18, pulsed lights 92, 94, and 96 emit lights at the period of T. In this case, as mentioned above, the period T is 1 kHz to several MHz, and the light emission time is about several (n) seconds, and then, only at the moment, light information from the target workpiece can be obtained. In addition, since the quantity of light changes no less than 30% per pulse, when the stage is continuously moved at the conventional stage speed, the measurement result of the light quantity has a large error, and then it is impossible to accurately inspect defects.
Now, the inspection region is assumed to be 10×10 cm. Regarding the number of light receiving elements, one is arrayed in the direction of the stage movement and two thousand are arrayed perpendicularly to the stage movement. One light receiving element is assumed to be able to measure a light quantity of the region of 100 nm×100 nm on the target workpiece. In this case, the frame width (at right angles to the direction of continuous movement of the stage) is 100 nm×2000 pieces=200 μm=0.2 mm. The number of frames is 10 cm/0.2 mm=500. It is herein assumed that the inspection time is suppressed to be two hours or twenty hours, for example, and in that case, the inspection time per frame of the stage is 14.4 seconds or 144 seconds. Since the frame length (direction of continuous movement of the stage) is 10 cm, the stage speed at this time is 10 cm/14.4 seconds=6.94 mm/second, or 10 cm/144 seconds=0.694 mm/second. On the other hand, when the oscillation frequency of the pulsed laser is assumed to be 40 kHz, the oscillation period is 1/40 kHz=0.025 msecond=25 μsecond. When a pulsed laser illuminates the target workpiece, the sensor measures the quantity of light of a certain position, and a next pulsed laser illuminates the target workpiece in 25 micro seconds, since the stage and the target workpiece are moving, their movement amount is 6.94 mm/second×25 micro seconds=173.5 nm or 0.694 mm/second×25 micro seconds=17.35 nm. The number of times of irradiation times of the pulsed laser used per pixel is 100 nm/173.5 nm=0.57 times in the former case, and 5.7 times in the latter case. As mentioned above, since the light quantity of each pulse changes about 30%, changes of the measured light quantity per pixel are 30%/√0.57=39.73% and 30%/√5.7=12.6% respectively, and they serve as measurement errors of the light quantity, which makes it difficult to pass the inspection.
Although the above example is the case of using a line sensor, another case of using the TDI sensor mentioned above is almost the same as the line sensor case as follows: When five light receiving elements are arrayed in a line in the direction of continuous movement of the stage, the number of irradiation times of a pulsed laser per pixel becomes five times. In this case, change of the measured light quantity per pixel becomes 30%/√(0.57×5)=17.77% and 30%/√(5.7×5)=5.62%, which also makes it difficult to pass the inspection.
Furthermore, in the above-mentioned example of suppressing the mask inspection time to be two hours, the stage moves 173.5 nm during pulses, that is a time period after a certain pulse illuminates until a following pulse illuminates. Since the size of a pixel is 100 nm, this stage movement corresponds to a movement for 1.735 pixels, which is greater than 1 pixel by 0.735 pixel. This means that the (relative) position of the sensor proceeds further than the pixel existing next to another pixel which illuminated previously, by 0.735 pixel. Consequently, in the case of using the line sensor, the light quantity of 0.735 pixel (73.5%) in the pixel existing next to another pixel previously illuminated cannot be measured by the line sensor. Naturally, it is impossible to judge the existence of defects in this region that cannot be measured, which is a fatal problem in inspecting defects. Furthermore, in the case of using the TDI sensor, the following problems arise. When the sensor moves by 1.735 pixels, the position of one light receiving element of the TDI sensor extends over the boundary between two pixels. Therefore, one light receiving element measures a light quantity composed of two light quantities, that is a light quantity of 0.27 pixel and a light quantity of 0.73 pixel. Since this distribution ratio changes depending upon a pulse generating timing, the conventional method cannot control it. Consequently, information on a light quantity measured by the TDI sensor is composed of light quantity information on two pixels, which is mixed without controlling, thereby deteriorating the measurement precision and reducing the capability of inspecting defects.
Thus, when the conventional method of using continuous light is applied to the case of pulsed lasers, there exists a critical defect of being unable to accurately judge defects.